Good amount of Tris for a mobile game?
As much as necessary but as few as possible :)

The goal is to have a playable performance. And this depends on too much factors like shaders, draw calls, how much geometry at once, rest of ongoing code, target platform, target OS, and and and. The geometry is usually the smallest problem nowadays. The common graphical resource hogs are draw calls, particle systems, shadows. But code can slow down the game dramatically too.

That's why you normally start with a prototype to figure out what is possible within the given bounds. So make a prototype and simply try it out.